This is a continuation-in-part of U.S. patent application Ser. No. 642,574, filed on Aug. 20, 1984 now abandoned.
This invention relates to a methane pyrolysis process for manufacturing graphite fibers of a type suitable for filler in plastic composites. More particularly, this invention relates to regulation of reactant methane concentration during sequential stages of fiber growth.
U.S. Pat. No. 4,391,787, issued July 5, 1983 and assigned to the present assignee, describes a method for manufacturing thin, straight graphite fibers by natural gas pyrolysis carried out within a thin-wall stainless steel tube surrounded by wet hydrogen gas. The fibers are preferably 5 to 15 microns in diameter and up to several centimeters long and are well suited for plastic filler. In this process, fiber growth is attributed to a complex interaction between the chromium-bearing steel and methane at elevated temperature. U.S. Pat. No. 4,491,569 describes a method for growing the fibers on a ceramic surface, but only after natural gas pyrolysis is initiated adjacent stainless steel. Stainless steel is relatively expensive and has a limited useful life at fiber growing temperatures. Furthermore, unavoidable carburization of the stainless steel unproductively removes carbon and reduces yield.
Fiber growth is believed to proceed in two sequential stages. During a first stage, pyrolytic carbon interacts with metallic nuclei to form long, thin filaments less than 100 nanometers in diameter. Suitable nuclei are derived from a deposit produced by evaporation of ferric nitrate solution, as described in U.S. patent application Ser. No. 496,482. Once nucleated, the filament grows rapidly in length. However, for reasons not fully understood, the filament then ceases to lengthen. Thereafter, additional pyrolytic carbon thickens the filament into a fiber that is several microns in diameter. It is believed that pyrolysis reactions initiated between the stainless steel and the natural gas in some manner control the nature and the concentration of carbonaceous precursors in the gas stream to favor first filament formation and then thickening.
Therefore, it is an object of this invention to provide an improved process for growing graphite fibers by natural gas pyrolysis on a ceramic surface that does not require that pyrolysis be initiated adjacent stainless steel.
More generally, it is an object of this invention to provide an improved process for growing graphite fibers by pyrolysis of a methane gas, wherein the methane concentration in the gas is regulated first at a value particularly conducive to reactions that nucleate and lengthen filaments and thereafter at a relatively richer value for thickening the filaments into fibers. Although other parameters such as temperature, gas flow rate and nuclei characteristics also affect fiber growth, regulation of the methane concentration during each step improves the overall process by increasing yield, maximizing length and reducing growing time.